Unlike previously expected, anthracycline antibiotics do not all concentrate in cell nuclei. Depending on their structures they are concentrated in cell nuclei, cell cytoplasm, or have no specific localization. Correlation of intracellular drug localization with anthracycline structure was made. Bleomycin acts catalytically to promote oxidation of ferrous ion. One iron binding site occurs on bleomycin and the Fe ions bleomycin complex can be reduced to Fe ions bleomycin by intracellular reductants.